Numerology's own literature is oddly reluctant to crown a single lucky number — every digit gets a strength and a caution, and ranking them by fortune would undercut that balance. So when a source does answer this question directly, it's usually leaning on something older than numerology: 7's long-standing status as a fortunate number in religious texts, folk sayings, and games of chance across many unrelated cultures.
That head start matters. A number doesn't need numerology's help to feel lucky if it's already spent millennia in that role — seven wonders, seven seas, a winning roll in dice games, a jackpot symbol on a slot machine. Numerology's own reading of 7 (insight, depth, quiet fortune) fits comfortably on top of that older reputation rather than competing with it.
Compare that to a number like 3, which numerology calls lucky more on its own terms — associated with joy and social ease — without much outside cultural reinforcement. Both get called "lucky," but 7's version of the claim is doing double duty, borrowing credibility it didn't generate itself.